IV. Retreating Shorelines
4
Impacts and Interaction
309
Man has manifested an irresistible urge to "correct" or "improve" on nature's
work. In his relentless efforts to establish ports, to straighten shorelines, to
"protect" beaches and cliffs, or to provide unimpeded view, and especially to
develop areas, he has erected structures in water and near the shoreline, on
barrier islands, beaches and dunes.
Vertical walls have been erected in many shore resorts, often to provide a
promenade. This increases wave energy dissipation by beach material fronting
the wall and augments erosion. The famed Promenade des Anglais at Nice is at
the origin of the loss of the entire sandy portion of the beach of the Baie des
Anges.
Engineering structures, whatever their nature or type, have nearly always
initiated, or aggravated, erosion in coastal areas. Strangely this has even occurred
where there is no longshore littoral transport. One of the earliest and most
intensive cases of jetty induced erosion was reported for four India east coast
harbors : Paradip, Vishakhapatnam, Kakinada and Madras. Madras is a harbor
developed on the open coast, like Zeebrugge in Belgium. No sooner had a
breakwater been constructed (1875) that groins had to be implanted to the north
side ; a beach built up on the southern side as rapidly as the construction itself.
Completed twenty years after its inception, the "southside" sand soon shoaled the
harbor entrance which was relocated northward and an outer quay built. The
blockage of the littoral drift resulted in deposition and erosion that extended over
many kilometres of shoreline, with maximal effects during the early construction
stages ; erosion reached 90 m (275 ft) when exacerbated by storm effects.
Detached breakwaters have equally severe results, witness those near Los
Angeles, at Santa Barbara (1927) and Santa Monica. Contrary to expectations
they too blocked littoral drift ; if wave energy is decreased by the structure, the
longshore transport is at the same time halted though there is no "damming"
across the nearshore. Furthermore, accumulation as a spit, on the updrift side
eventually reaches the tip of the breakwater, goes still further, and starts shoaling
the harbor entrance. Dredging in the only remedy to keep the port's access open.
At least, at Santa Barbara, the material is not lost to the littoral system, as it is
dumped as replenishment on the downdrift "starved" beach.
Cross-bay and cross-estuary jetties also dam the littoral drift updrift, while
downdrift the current carries sand away from the jetty, thus causing shore
erosion. A classical case is that of Ocean City MD.
4
Impacts and Interaction
309
Man has manifested an irresistible urge to "correct" or "improve" on nature's
work. In his relentless efforts to establish ports, to straighten shorelines, to
"protect" beaches and cliffs, or to provide unimpeded view, and especially to
develop areas, he has erected structures in water and near the shoreline, on
barrier islands, beaches and dunes.
Vertical walls have been erected in many shore resorts, often to provide a
promenade. This increases wave energy dissipation by beach material fronting
the wall and augments erosion. The famed Promenade des Anglais at Nice is at
the origin of the loss of the entire sandy portion of the beach of the Baie des
Anges.
Engineering structures, whatever their nature or type, have nearly always
initiated, or aggravated, erosion in coastal areas. Strangely this has even occurred
where there is no longshore littoral transport. One of the earliest and most
intensive cases of jetty induced erosion was reported for four India east coast
harbors : Paradip, Vishakhapatnam, Kakinada and Madras. Madras is a harbor
developed on the open coast, like Zeebrugge in Belgium. No sooner had a
breakwater been constructed (1875) that groins had to be implanted to the north
side ; a beach built up on the southern side as rapidly as the construction itself.
Completed twenty years after its inception, the "southside" sand soon shoaled the
harbor entrance which was relocated northward and an outer quay built. The
blockage of the littoral drift resulted in deposition and erosion that extended over
many kilometres of shoreline, with maximal effects during the early construction
stages ; erosion reached 90 m (275 ft) when exacerbated by storm effects.
Detached breakwaters have equally severe results, witness those near Los
Angeles, at Santa Barbara (1927) and Santa Monica. Contrary to expectations
they too blocked littoral drift ; if wave energy is decreased by the structure, the
longshore transport is at the same time halted though there is no "damming"
across the nearshore. Furthermore, accumulation as a spit, on the updrift side
eventually reaches the tip of the breakwater, goes still further, and starts shoaling
the harbor entrance. Dredging in the only remedy to keep the port's access open.
At least, at Santa Barbara, the material is not lost to the littoral system, as it is
dumped as replenishment on the downdrift "starved" beach.
Cross-bay and cross-estuary jetties also dam the littoral drift updrift, while
downdrift the current carries sand away from the jetty, thus causing shore
erosion. A classical case is that of Ocean City MD.